Node.js with TypeScript

Setting up a Node.js app with TypeScript, Nodemon, and Mocha/Chai tests


Header Image

The idea with this article is to simply provide a quick tutorial on how to set up a simple Node.js project with nodemon for development reloading and of course unit tests. This means I have gone for a minimalist approach and tried to keep everything as generic as possible by not doing things like sticking to my preference of Yarn or adding unnecessary dependencies.

Let’s start off the same way we start any node project, running npm init in our project’s folder.

npm init

When prompted by npm to answer the questions you have the option of just pressing enter every time and filling out your package.json later, but I took this opportunity to sneak in our test command:

mocha -r ts-node/register test/**/*.spec.ts

Here we get a glimpse of the tool that is helping us achieve this whole thing; ts-node provides TypeScript execution for Node.js, and here it allows us to execute mocha tests that were written in TypeScript.

You will also notice that I set our main option to index.ts instead of index.js, later on this will tell tools like nodemon where to look for our starting point.

ScreenShot of npm init output

Next we need to set up TypeScript in the project

npm i typescript

While we are here, let’s also install ts-node:

npm i -D ts-node

You have the option of also installing TypeScript globally but I chose to run the locally installed module for the next step using npx.

So, lets initialise our TypeScript project.

npx tsc --init

We should now have our default tsconfig.json file

Something to execute

In this next bit we need to write some TypeScript to execute. I have gone for a simple express app here in index.ts:

import express, {Express} from 'express'

const port: number = 3000
const app: Express = express()

app.get('/', (req, res) => {
    res.json({message: 'Hello World'})

app.listen(port, () => {
    console.log(`Express is listening on port ${port}`)

At this point if we run the app having only installed the express module (npm i express ):

npx ts-node index.ts

We will get the following error:

Screenshot of npx ts-node index.ts output with error

This is because the default tsconfig.json has strict type checking enabled, removing strict”: true would indeed fix this error, but since you are going through the effort of setting up TypeScript, you probably really want type safety. So the other option is to install the types for any packages you may need, in this example, for express:

npm i @types/express

Now when we run npx ts-node index.ts we should get a running express server:

Screenshot of npm ts-node index.ts output

We don’t however want to be running our app via ts-node in production, so let’s set up a start script that will build our JavaScript and run our app in the regular Node.js environment.

Currently if we had to run tsc it would output our index.js right next to our source code. Some people prefer it this way and that’s completely fine, but my personal preference is to have output go to a build directory, we can do this by setting the outDir property in our tsconfig.json:

    "outDir": "./build",

We can now set up our build and start scripts in our package.json:

    "scripts": {
        "build": "tsc",
        "start": "tsc && node ./build/index.js",
        "test": "mocha -r ts-node/register test/**/*.spec.ts"

Unit Testing

Now that we have a running TypeScript app, we want to set up some unit testing. Since we already set up mocha with ts-node in the beginning we should be able to simply run it once written, you don’t have to just use mocha though, ts-node also officially supports tape, and you can probably get it to run other tests as well.

Lets start off by installing our dependencies, remembering to also install the typings for mocha and chai:

npm i -D mocha chai @types/mocha @types/chai

Then we create a simple test (I called mine default.spec.ts) inside a folder named test:

import {expect} from 'chai'

describe('When we run tests', () => {
    it('Should actually work' , () => {

This certainly isn’t a useful test but this isn’t a unit testing tutorial, we just want to see that test’s run in our setup. Let’s run npm test:

Screenshot of npm test output

Neat! our test’s work.


Nodemon is a neat tool that will run and reload your application when the source code changes. Honestly once you work with a tool like this you want it everywhere because it comes in really handy during development.

First, lets install nodemon

npm i -D nodemon

Next we configure nodemon to work with our TypeScript files. Nodemon can be configured either as a property inside your package.json or in it’s own nodemon.json file, which is the method I chose to use here:

    "ext": "ts",
    "exec": "npm start"

Pro Tip: The nodemon config supports an env property so you can set environment variables, I use this to set environment variables for dev that will be different in production, ie: “env”: { “DEBUG”: “app,app:*” }

Now if we simply run npx nodemon our app will restart when we make code changes, go add a line to your index.ts and watch the magic!

And that’s a wrap! You should now have a nicely set up TypeScript Node.js application. Enjoy the type safety!